The invention relates to methods and apparatus for handling digital representations of documents for example to allow sophisticated computer aided design and manufacture of a security document.
Conventional commercial colour proofing relies on the proofing media having yellow, magenta, cyan and black halftone screened colour separations which match the YMCK inks that will be used for printing. Each ink naturally has to be applied from a printing plate which contains the Y, M, C or K information.
The proofs are generally made by exposing Y, M, C and K photosensitive media to the corresponding photographic halftone colour separations, using a contact printing method. The individual colour layers are then mechanically overlaid in register.
Computer generated proofing methods are available for commercial work. One method requires Y, M, C or K halftone photographic separation films to be made which are in turn used as masks for contact imaging or proofing media. Another method exposes three or four colour sensitive media.
Problems arise with the printing of security documents. Firstly they involve different types of printing processes in the manufacture of any one document whether, say, a banknote, passport or identity card. Secondly usually one of the printing processes is different from those generally used in the printing industry or is conducted in an unusual manner e.g. intaglio printing. The printing methods often apply images only with a partial contact of the plate (such as through the use of schablones) or the inks are delivered in a special manner such as by rainbow printing from split ducts.
It is important to appreciate that most security documents are printed in continuous tone often using continuously blended colours, the blending occuring only on the ink train of the press. Security documents often make use of fine line patterns, some delivered in such "rainbow" printing fashion.
In contrast conventional full colour printing (e.g. magazine printing) will take a colour original, colour separate that, convert it into a halftone format and then print with the three subtractive colours and black. These subtractive inks are printed at full density or not at all (ie. they are not printed at continuous density levels) and their overall appearance depends on the eye blending together interspersed finely divided areas of the subtractive colours and black to give the perceived colour. This combination thus creates the perception of the original. It is however an illusion, generally unacceptable in security printed items.
Although not generally noticeable to the naked eye there has to be a sacrifice of the ultimate level of resolution because of the screening process. While coarse colour pictures of bank notes could be attempted this way, they would not survive customer's inspection and it is extremely important that the proofed security document should have a resolution similar to the final article--which will be much better than half tone processes allow. It is also important to remember that the inks delivered to the security document are not selected from four or six pots and only delivered in those colours. Rainbow printing blends the colours on the press to give continuous hues as well as continuous density.
The methods and materials used in the security printing industry are such that genuine documents can be verified such as by visual or machine authentication methods, and counterfeits or forgeries detected. The computer aided design system employed for security documents must be capable of providing visual representations to an unprecedented degree of accuracy of colour and resolution so that proofing prints can approximate more closely to the manufactured counterpart. In particular the design system must be able to simulate the effect of rainbow printing. Additionally it is highly desirable that the design system is able to provide visual indications of the placement of special markings such as invisible but ultraviolet revealable inks and magnetic inks.
For example a typical banknote will be printed to give a notional printing structure of [from the front face downwards]:
______________________________________ Serial numbering; monochrome or multicolour/letterpress; Security design; rainbow, monochrome or multicolour/intaglio; FRONT Security design; invisible fluorescent/litho or FACE letterpress; Security design; rainbow or monochrome/litho; Security design; rainbow or monochrome/litho; Security design; rainbow or monochrome/litho; . . . Security design; rainbow or monochrome/litho; Base paper with optional watermark, threads, . . . marking fibres; Security design; rainbow or monochrome/litho; BACK Security design; rainbow or monochrome/litho; FACE Security design; rainbow or monochrome/litho; Security design; rainbow or monochrome/litho; Security design; invisible fluorescent/litho or letterpress; Security design; rainbow, monochrome or multicolour/intaglio. ______________________________________
The printing impressions are placed in the above sequence from the base paper outwards. There need be no overlap of one impression on another as often only portions of the note are printed with a given plate. Typically six litho workings will be applied to the document. Finally the serial number is added by letterpress.
In practice there may be between two and eight lithographic impressions, up to four on each side. Cheques and other security printed items need not be printed by intaglio but generally will have at least three ink impressions on one side.
The intaglio impression on each side is generally a single impression and one plate is used. This plate however is selectively inked by the use of areas termed "schablones" which need to be cut to size according to the colour to be delivered to the design feature. These schablones may supply monochrome, multicolour or rainbow coloured ink.
Computer aided design systems which could hitherto be applied to security printing have been limited in usefulness being object or vector based. Vector based systems are very useful for producing a component of a complete document design such as complex line patterns. Vector based systems do not readily extend to the introduction of a variety of colour changes with a given mathematically constructed feature nor can they be useful for creating images of a complete document.
The LaserScan High Resolution Display device is a vector operated system in which complex line images are written by a laser onto a photochromic film which becomes opaque where the laser strikes it. The resulting image is then projected onto a screen for viewing; the image is not displayed on a CRT screen at that resolution. Such systems are commonly used in present day security document design and any new design system has to be able to import vector patterns from it.
In object based design systems the user specifies the composition of the image as a series of objects such as lines, circles, polygons which are assigned a position, size and colour. The objects can be manipulated singly or in groups and ranked according to whether they are to appear behind or in front of one another. To display the image the attributes of each object are processed though a programme which computes the colour to be assigned to each pixel of the display, loading the appropriate numbers into the framestore.
Object based design systems have the disadvantage that they do not provide the artist with natural working methods; they cannot work with scanned images and the refresh time for complex images is long.